Observationally speaking, the Fe-L complex is both a blessing and a curse. On one hand, as mentioned above, it involves modelling the emission of Fe ions with many remaining electrons, which is significantly more difficult than H- or He-like transitions. On the other hand, because these lines are very bright, the total 0.5–2.0 keV flux of a kT=0.6–0.8 keV IGrM plasma with a Solar composition is more than three times brighter than a 4 keV ICM plasma with the same emission measure (defined as \(\int n_e n_H dV\) ). Quite literally, metals make the IGrM shine, and emission from Fe in particular is crucial for detecting what would otherwise be extremely faint and diffuse plasma in galaxy groups. However, this also means that, unlike the case of the ICM, a knowledge of the metallicity in the IGrM is indispensable for converting the observed X-ray flux into a particle number density. Furthermore, since the total flux and shape of the Fe-L bump are extremely sensitive to the plasma temperature (e.g. see the difference between the Fe-L model for a 0.6 vs. a 1.2 keV plasma in Fig. REF ), significant biases arise when a spectrum containing multiple temperature components (for instance due to projection effects or radiative cooling in the core of a group) is approximated by a single-temperature model. This effect, dubbed `the Fe bias', is discussed in detail already in [1]} and [2]} using ASCA data of elliptical galaxies and galaxy groups.
In that work, and many subsequent references thereto, it is consistently shown that, if the abundance measurements are driven by the Fe-L signal, a two-temperature model can yield best-fit metallicities more than twice higher than a single temperature approximation. Although this conclusion was originally reached with old versions of the Fe-L plasma emission, it remains true today, as we illustrate in Fig. REF .
It is also worth mentioning in passing that, at least at CCD spectral resolution, multi-temperature models can only be constrained if the abundances of the two components are coupled to each other, which need not be true in nature. In addition, beside the IGrM being intrinsically multi-phase, the unresolved emission from low-mass X-ray binaries (LMXB) may be an important spectral component, at least near the center of the brightest group galaxy (BGG) [3]}; if unaccounted for, this may lead to biases in the measured abundances as well.
